In recent years, builders have been paying more attention to
the importance of keeping water out of walls. News of
construction-defect lawsuits in California, EIFS failures in
North Carolina, and the "leaky condo" crisis in British
Columbia have all driven home the point that leaking buildings
can cause major headaches for builders. One result of the focus
on waterproof walls is the growing use of peel-and-stick
membranes and other types of flexible flashing.
The term "flexible flashing" is used to describe a broad
category of nonmetallic flashings, including both
peel-and-stick and nonstick flashings. Manufacturers have not
yet agreed on a generic term for these products, which are
referred to as self-adhering bituminous tapes, flashing tapes,
waterproofing tapes, flexible window flashings, flashing
membranes, and wall tapes.
Only in the past few years have these flashings become common
on residential job sites. Flexible flashing is rapidly
replacing traditional felt splines for sealing the perimeter of
finned windows. Some flexible-flashing manufacturers promote
the use of these products at other locations, as well: to cover
below-grade concrete cracks, at roof penetrations, under
exterior door sills, over deck ledger boards, at inside and
outside corners of wall sheathing, under stucco shelves and
parapets, and over sections of wall sheathing susceptible to
splashback. But by far the most common use of flexible flashing
is at window and door perimeters.
These new materials have some significant advantages over
traditional flashing materials. Unlike most metal flashings,
for example, peel-and-stick flashings conform easily to unusual
shapes. Most types of flexible flashing can be folded to form a
waterproof end-dam on a rough windowsill, where making the same
shape with copper would require soldering the flashing at the
corners. Manufacturers claim that peel-and-stick flashings,
unlike metal flashing, can form a waterproof seal between the
flashing and the substrate.
These flashings are versatile and easy to install. But before
slapping peel-and-stick over every exterior crack, you need to
be sure you've chosen the right product for a given
application. It's also important to know about potential
compatibility problems and to avoid accidentally creating a
wrong-side vapor barrier.
Most peel-and-stick flashings are made from rubberized asphalt,
also known as modified asphalt, modified bitumen, or rubberized
bitumen. Rubberized-asphalt membranes were originally developed
to protect roofs from ice dams. As builders recognized new uses
for the product, several manufacturers began selling it in
narrow rolls — typically between 4 and 12 inches wide
— for a variety of flashing applications (Figure 1).
Self-adhering rubberized-asphalt flashings are made of
the same material as the eaves membranes used to
prevent ice dam leaks.
Rubberized asphalt used for flashing is made by modifying
asphalt with styrene butadiene styrene (SBS), which makes the
asphalt more rubber-like. SBS-modified asphalt, being elastic,
can accommodate thermal expansion and contraction in building
components. Because of its "cold flow" characteristics,
rubberized asphalt can also seal around fastener
Sticky stuff. As long as the
surface is clean and warm, rubberized asphalt sticks to a wide
variety of substrates: dimensional lumber, plywood, steel,
aluminum, hard vinyl, asphalt felt, and plastic housewrap
(Figure 2). Some manufacturers of rubberized-asphalt flashing
advise that their products may not stick well to concrete,
masonry, or OSB unless these substrates are first primed.
Rubberized-asphalt flashings, like Grace's Vycor Plus,
stick well to unprimed plywood. Some manufacturers of
rubberized-asphalt flashings warn that adhesion to OSB
can be difficult unless the OSB is first primed.
To make it possible to handle such a sticky substance, one
side of the rubberized asphalt is laminated to a thin sheet
(usually about 8 mils) of cross-laminated high-density
polyethylene, and the other side is protected with a
siliconized paper release sheet. Instead of polyethylene, some
manufacturers laminate a thin layer of aluminum foil to the top
of their rubberized-asphalt flashings (Figure 3).
Figure 3. Since
foil-topped flashings like Peel'N'Stick from Polyguard
Products can be left exposed to the weather for a
longer period than polyethylene-topped flashings, they
are a good choice when siding installation may be
Stickiness is a double-edged sword. In warm temperatures,
when rubberized asphalt is at its stickiest, it can be
impossible to readjust a flashing once it has touched a
Keep it covered.
Rubberized-asphalt flashings, except for those laminated with
aluminum foil, should not be left exposed to the weather.
Eventually, ultraviolet light breaks down the polyethylene,
exposing the modified asphalt, which then begins to oxidize.
Most manufacturers recommend that their flashings be covered
within 30 days of installation, although one manufacturer,
Protecto Wrap, says that its BT20XL Building Tape can be left
exposed for up to 120 days.
Several manufacturers make peel-and-stick flashings from
butyl, also called butyl rubber (Figure 4). Butyl flashings are
usually black, resembling their rubberized-asphalt cousins.
However, butyl flashings lack the asphalt smell that
distinguishes rubberized-asphalt products, and they feel more
rubbery. Like rubberized-asphalt flashings, butyl flashings are
available with a top surface of either polyethylene or aluminum
foil. Those with a top surface of polyethylene should not be
left permanently exposed to the weather. FlexWrap, a butyl
flashing from DuPont, has a top layer of corrugated Tyvek that
enables it to conform to curved shapes, like the heads of
Self-adhering butyl flashings, like rubberized-asphalt
flashings, can have a top layer of either polyethylene
or aluminum foil. Butyl flashing, although more
expensive than rubberized-asphalt flashings, can be
installed over a wider temperature range.